The ability of the immune system to distinguish self from non-self is critical in enabling it to neutralize pathogens while remaining non-responsive (i.e. tolerant) of the body's own tissues. Thus, when immune tolerance to self fails, the development of autoimmune disease can result. Interestingly, the mechanisms that mediate tolerance induction to self-antigens also appear to prevent immune responses against tumors. Thus, the induction of anti-tumor immunity elicited upon vaccination appears to be analogous to the breakdown of tolerance to self- antigens that results in autoimmunity. Therefore, understanding the pathways by which immune tolerance to self-antigens is induced, as well as how these pathways can be manipulated, will be helpful in developing strategies to manage diseases in which it is either desirable to induce or reestablish tolerance (i.e. autoimmunity) as well as to block or reverse tolerance (i.e. cancer). To study how immune tolerance in T lymphocytes is established for self-antigens expressed in a variety of parenchymal tissues, transgenic mice expressing the model self-antigen hemagluttinin (HA) under the control of the rat C3(1) promoter have been generated. When naive HA-specific CD4+ T cells are adoptively transferred into these transgenic mice expressing HA, they become functionally tolerant as evidenced by their inability to respond to subsequent stimulation with HA. The naive CD4 cells are not induced into a tolerant state through direct contact with the HA-expressing parenchymal cells; rather, bone marrow-derived APCs that acquire HA present the class II-restricted epitope in a toleragenic manner to the naive CD4 cells. The experiments outlined in this application are aimed at characterizing the APCs that mediate this "cross-tolerance" pathway (i.e. the tAPC), and in a more general sense, defining the parameters that confer toleragenic versus immunogenic antigen-presenting function. The specific aims are to: 1) identify the tAPC; 2) characterize functional properties of the tAPC; and 3) analyze how the tAPC deliver its toleragenic signal.